You also have to have implementation in the network code as well as the kernel. That said, (K)Ubuntu has supported it for a few years, at least, and every major distribution I have looked at has supported it since 2004 (at the latest). Special, niche distros may not have support, but I don not know which those would be.
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Joshua NurczykJun 9 '09 at 18:27

On 1996 the IPv6 support start in Linux kernel development version 2.1.8 and on 2005, Linux 2.6.12 removes experimental status from its IPv6 implementation.
So the IPV6 module is normally autoloaded on most recent Linux distributions default kernels (2.6.x)

Mr. Volkerding (the head Slackware maintainer) is known for including older software in Slackware on the basis that it's more stable. Slackware officially made the shift to 2.6.x in mid 2007 with the 12.0 release.
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andrewd18Jun 9 '09 at 18:29